Aaron Greenberg first replied that as per his knowledge, Forza 5 and Ryse runs at native 1080p. But there seems to be some mistake in these figure, and Aaron has just updated his reply with the following statement:

"Update from team, Forza 5 is native @1080p, Ryse is native @900p, but Xbox One upscales all titles to 1080p & they look amazing"